Rails has_many relationship with prefilled views - ruby-on-rails

Question

I have a pretty basic Rails 4 app, and am using Cocoon's nested forms to manage the has_many... :through model association.
class Student < ActiveRecord::Base
has_many :evaluations
has_many :assessments, through: :evaluations
# ... etc
end
class Evaluation < ActiveRecord::Base
belongs_to :student
belongs_to :assessment
# ... etc
end
class Assessment < ActiveRecord::Base
has_many :evaluations
has_many :students, through: :evaluations
accepts_nested_attributes_for :evaluation, reject_if: :all_blank
# ... etc
end
When I use Cocoon in the View, I want to use the New Assessment view to pre-fill all the Student records in order to create a new Evaluation for each one. I don't want to have to do some hacky logic on the controller side to add some new records manually, so how would I structure the incoming request? With Cocoon I see that requests have some number in the space where the id would go (I've replaced these with ?? below).
{"utf8"=>"✓", "authenticity_token"=>"whatever", "assessment"=>{"description"=>"quiz 3", "date(3i)"=>"24", "date(2i)"=>"10", "date(1i)"=>"2015", "assessments_attributes"=>{"??"=>{"student_id"=>"2", "grade" => "A"}, "??"=>{"student_id"=>"1", "grade" => "B"}, "??"=>{"student_id"=>"3", "grade"=>"C"}}, }}, "commit"=>"Create Assessment"}
I see in the Coccoon source code that this is somehow generated but I can't figure out how it works with the Rails engine to make this into a new record without an ID.
What algorithm should I use (or rules should I follow) to fill in the id above to make a new record?

Answer 1

"??"
Never a good sign in your params.
With Cocoon I see that requests have some number in the space where the id would go
That ID is nothing more than the next ID in the fields_for array that Rails creates. It's not your record's id (more explained below).
From your setup, here's what I'd do:
#app/models/student.rb
class Student < ActiveRecord::Base
has_many :evaluations
has_many :assessments, through: :evaluations
end
#app/models/evaluation.rb
class Evaluation < ActiveRecord::Base
belongs_to :student
belongs_to :assessment
end
#app/models/assessment.rb
class Assessment < ActiveRecord::Base
has_many :evaluations
has_many :students, through: :evaluations
accepts_nested_attributes_for :evaluations, reject_if: :all_blank
end
This will allow you to do the following:
#app/controllers/assessments_controller.rb
class AssessmentsController < ApplicationController
def new
#assessment = Assessment.new
#students = Student.all
#students.each do
#assessment.evaluations.build
end
end
end
Allowing you:
#app/views/assessments/new.html.erb
<%= form_for #assessment do |f| %>
<%= f.fields_for :evaluations, #students do |e| %>
<%= e.hidden_field :student_id %>
<%= e.text_field :grade %>
<% end %>
<%= f.submit %>
<% end %>
As far as I can tell, this will provide the functionality you need.
Remember that each evaluation can connect with existing students, meaning that if you pull #students = Student.all, it will populate the fields_for accordingly.
If you wanted to add new students through your form, it's a slightly different ballgame.
Cocoon
You should also be clear about the role of Cocoon.
You seem like an experienced dev so I'll cut to the chase - Cocoon is front-end, what you're asking is back-end.
Specifically, Cocoon is meant to give you the ability to add a number of fields_for associated fields to a form. This was discussed in this Railscast...
Technically, Cocoon is just a way to create new fields_for records for a form. It's only required if you want to dynamically "add" fields (the RailsCast will tell you more).
Thus, if you wanted to just have a "static" array of associative data fields (which is I think what you're asking), you'll be able to use fields_for as submitted in both Max and my answers.

Answer 2

Thanks to #rich-peck I was able to figure out exactly what I wanted to do. I'm leaving his answer as accepted because it was basically how I got to my own. :)
assessments/new.html.haml (just raw, no fancy formatting)
= form_for #assessment do |f|
= f.fields_for :evaluations do |ff|
.meaningless-div
= ff.object.student.name
= ff.hidden_field :student_id, value: ff.object.student_id
= ff.label :comment
= ff.text_field :comment
%br/
assessments_controller.rb
def new
#assessment = Assessment.new
#students = Student.all
#students.each do |student|
#assessment.evaluations.build(student: student)
end
end